This invention relates to a circuit which is readily adapted to detect an angle-modulated signal and, more particularly, to a quadrature-type phase detector which is operative as an FM detector and which exhibits superior AM suppression characteristics. The circuit of this invention advantageously can be fabricated as an integrated circuit.
A quadrature-type phase detector generally is a switching or gating circuit which, when supplied with amplitude-limited angle-modulated signals, such as a frequency modulated (FM) signal, is capable of detecting the information signal which was used to modulate the angle-modulated signal. One advantageous use of a quadrature-type phase detector is as an FM detector.
One example of a typical prior art quadrature type phase detector is described in U.S. Pat. No. 3,548,326 to Bilotti. In this patented circuit, a plural-stage amplifier limiter is supplied with an FM signal and produces an amplitude-limited version of that FM signal. The limited FM signal is supplied as a reference signal to one input of a detector stage. This same amplitude-limited FM signal also is supplied through a phase shifter, so as to be shifted by 90.degree., to a pair of switching stages, such as gating circuits, included in the detector stage. The phase of the 90.degree. phase-shifted reference signal, which is the quadrature signal, differs in phase with respect to the reference signal in accordance with the frequency modulation of the input signal. The currents flowing through the gating circuits of the detector stage are determined by the conductivity of differentially connected transistors, the latter being driven by the reference signal. Consequently, the outputs of the respective gating circuits are current pulses having respective durations which are proportional to the phase difference between the quadrature signal and the reference signal. These pulse durations are integrated so as to provide an amplitude envelope proportional to the original information signal which was used as the frequency-modulating signal.
One problem which is found with this prior art circuit is the apparent requirement that the plural-stage amplifier limiter is direct-coupled to the detector stage. This means that any DC drift in the amplifier limiter is coupled directly to the detector stage, resulting in an undesired DC drift in the latter stage, thereby causing the overall circuit to be unstable. Furthermore, because of this direct-coupling between the amplifier limiter and the detector stage, if the amplitude of the reference signal changes because of the operation of the amplifier limiter, the resultant amplitude modulated (AM) component will not be adequately suppressed, thereby producing errors in the output signal produced by the detector stage. Stated otherwise, the suppression ratio in this prior art circuit is not of a desirably high value because of the direct coupling between the amplifier limiter and the detector stage.